In direct sequence spread spectrum communication, before data detection, the carrier frequency offset and spreading code timing offset must be corrected to synchronize to the carrier and the spreading code. Sliding correlator has been used in prior art to correct the spreading code timing offset. However, such a way is time consuming and cannot be effectively down before carrier recovery because the received signal is affected by carrier offset.
Another method uses a matched filter for code despreading and then use a phase-lock loop to estimate the carrier frequency offset. However, the application of this prior method is limited to the condition that the carrier frequency offset is smaller than the reciprocal of the period of the spreading code. In fact, in direct sequence spread spectrum communication, the carrier frequency offset is generally larger than the reciprocal of the period of the spreading code, and thus the prior method is hardly applicable.
U.S. Pat. Nos. 4,601,005 and 4,998,111 provide a method of matching the fast Fourier transform (FFT) of spreading code with the FFT of received signals. The method of the two patents is designed for the global positioning system (GPS), and utilizing the way of increasing the points of FFT calculations of a matched filter to improve the precision of the estimate of carrier frequency offset. Such a method is practicable for data transmission rate of GPS signals of 50 bits/sec. However, the data transmission rate of a general direct sequence spread spectrum communication system, such as a code division multiple access (CDMA) system, is far larger than 50 bits/sec. Therefore, the above-mentioned method is not practicable in a general direct sequence spread spectrum communication system. Besides, the above-mentioned patents both use the output maximum peak of a matched filter as the starting point of a spreading code because there is only one main path in satellite communication channel and thus only one maximum peak will be generated. However, the transmission channel of a general direct sequence spread spectrum communication system usually has multiple paths, for embodiment, the communication channel of a CDMA system is a multipath fading channel, and thus the output maximum peak of a matched filter may not be the first-path of the channel.
Based on the disadvantages of the above-mentioned prior art and the problems associated therewith, an object of the present invention is to provide a method and apparatus for correcting carrier frequency offset in a general direct sequence spread spectrum communication system.
Another object of the present invention is to provide a method and apparatus for correcting both the carrier frequency offset and spreading code timing offset of a general direct sequence spread spectrum communication system.
A further object in the present invention is to provide a method and apparatus for the correction of carrier frequency offset in all direct sequence spread spectrum communication systems and timing offset of spreading code in said systems.
Again, a further object of the present invention is to provide a method and apparatus for wholly digitized correction of carrier frequency offset and spreading code timing offset in a direct sequence spread spectrum system, so as to be implemented with integrated circuit technology to achieve a small and compact design.